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Abstract:

An optical encoder equipped with an origin detection apparatus has a scale
provided with an optical grating, a plurality of light receiving elements
that is provided in association with the pitch of the optical grating and
movable relative to the scale and a light source that illuminates the
light receiving elements with light through the scale. An optically
discontinuous portion is provided in the optical grating of the scale, a
change of a light beam that occurs over a certain length of section at
the time when a light beam corresponding to the discontinuous portion is
incident on the light receiving elements, a change occurring in that
section is detected, calculation is performed, and an origin position is
detected from the result of the calculation.

Claims:

1-5. (canceled)

6. An optical encoder provided with an origin detection unit, comprising:a
scale having an optical grating portion provided with an optically
discontinuous portion;a light receiving unit including a plurality of
units of light receiving elements, each of the units of the light
receiving elements having the same number of light receiving elements,
the light receiving unit being provided in association with a pitch of
the optical grating and movable relative to the scale;a calculation unit
that detects a change of a signal from the light receiving unit over a
predetermined length of section that occurs when a light beam, which
illuminates the discontinuous portion of the scale and is reflected
thereon, is incident on the light receiving unit, performs calculation
based on the change, and detects an origin position based on the result
of the calculation,wherein in one cycle of signals which are output from
the light receiving unit and have a predetermined interval which is
determined based on a relationship between the number of units of the
light receiving elements in the light receiving unit and the size of the
discontinuous portion of the scale, either a middle voltage of a section
during which a detection signal changes from a minimum value to a maximum
value or a middle voltage of a section during which the detection signal
changes from a maximum value to a minimum value is obtained, andwherein
the middle voltage is retrieved every time when the scale moves by one
pitch, the origin position is detected by a calculation based on two
middle voltages apart from each other by "(a number of units in the light
receiving unit)+n-1" pitches, where n represents a width of the
discontinuous portion in terms of a number of light-and-shade slits on
the scale.

7. An optical encoder provided with an origin detection unit, comprising:a
scale having an optical grating portion provided with an optically
discontinuous portion;a light receiving unit including a plurality of
units of light receiving elements, each of the units of the light
receiving elements having the same number of light receiving elements,
the light receiving unit being provided in association with a pitch of
the optical grating and movable relative to the scale;a calculation unit
that detects a change of a signal from the light receiving unit over a
predetermined length of section that occurs when a light beam, which
illuminates the discontinuous portion of the scale and is reflected
thereon, is incident on the light receiving unit, performs calculation
based on the change, and detects an origin position based on the result
of the calculation,wherein in one cycle of signals which are output from
the light receiving unit and have a predetermined interval which is
determined based on a relationship between the number of units of the
light receiving elements in the light receiving unit and the size of the
discontinuous portion of the scale, either a middle voltage of a section
during which a detection signal changes from a minimum value to a maximum
value or a middle voltage of a section during which the detection signal
changes from a maximum value to a minimum value is obtained, andwherein
the middle voltage is retrieved every time when the scale moves by one
pitch, the origin position is detected by a calculation based on two
middle voltages apart from each other by "(a number of units in the light
receiving unit)-n+1" pitches, where n represents a width of the
discontinuous portion in terms of a number of light-and-shade slits on
the scale.

8. An optical encoder provided with an origin detection unit, comprising:a
scale having an optical grating portion provided with an optically
discontinuous portion;a light receiving unit including a plurality of
units of light receiving elements, each of the units of the light
receiving elements having the same number of light receiving elements,
the light receiving unit being provided in association with a pitch of
the optical grating and movable relative to the scale;a calculation unit
that detects a change of a signal from the light receiving unit over a
predetermined length of section that occurs when a light beam, which
illuminates the discontinuous portion of the scale and is reflected
thereon, is incident on the light receiving unit, performs calculation
based on the change, and detects an origin position based on the result
of the calculation,wherein in one cycle of signals which are output from
the light receiving unit and have a predetermined interval which is
determined based on a relationship between the number of units of the
light receiving elements in the light receiving unit and the size of the
discontinuous portion of the scale, both a middle voltage of a section
during which a detection signal changes from a minimum value to a maximum
value and a middle voltage of a section during which the detection signal
changes from a maximum value to a minimum value are obtained, andwherein
the middle voltage is retrieved every time when the scale moves by one
pitch, the origin position is detected by a calculation based on two
middle voltages apart from each other by "2.times.(a number of units in
the light receiving unit)+n-1" pitches, where n represents a width of the
discontinuous portion in terms of a number of light-and-shade slits on
the scale.

9. An optical encoder provided with an origin detection unit, comprising:a
scale having an optical grating portion provided with an optically
discontinuous portion;a light receiving unit including a plurality of
units of light receiving elements, each of the units of the light
receiving elements having the same number of light receiving elements,
the light receiving unit being provided in association with a pitch of
the optical grating and movable relative to the scale;a calculation unit
that detects a change of a signal from the light receiving unit over a
predetermined length of section that occurs when a light beam, which
illuminates the discontinuous portion of the scale and is reflected
thereon, is incident on the light receiving unit, performs calculation
based on the change, and detects an origin position based on the result
of the calculation,wherein in one cycle of signals which are output from
the light receiving unit and have a predetermined interval which is
determined based on a relationship between the number of units of the
light receiving elements in the light receiving unit and the size of the
discontinuous portion of the scale, both a middle voltage of a section
during which a detection signal changes from a minimum value to a maximum
value and a middle voltage of a section during which the detection signal
changes from a maximum value to a minimum value are obtained, andwherein
the middle voltage is retrieved every time when the scale moves by one
pitch, the origin position is detected by a calculation based on two
middle voltages apart from each other by "2.times.(a number of units in
the light receiving unit)-n+1" pitches, where n represents a width of the
discontinuous portion in terms of a number of light-and-shade slits on
the scale.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to an origin detection method for an
optical encoder that is used in displace measurement or angle
measurement.

[0003]2. Description of the Related Art

[0004]A photoelectric encoder has a main scale on which a first optical
grating is provided and an index scale opposed thereto on which a second
optical grating is provided. The photoelectric encoder further has a
light source that illuminates the main scale with light and a light
receiving element that receives light having been transmitted through or
reflected by the optical grating of the main scale and transmitted
through the optical grating of the index scale.

[0005]Japanese Patent Publication No. H06-056304 teaches use of a light
receiving element array that also functions as an index scale in a
photoelectric encoder of the above described type. The inventors of the
present invention have also filed patent applications, e.g. Japanese
Patent Application Laid-Open No. 2003-161645, on inventions concerning
encoders of the above described type.

[0006]The encoder having the above described structure is called an
incremental type encoder. This type of encoder detects the movement
amount of a scale by counting the number of output pulses generated by
movement of the scale. A problem encountered with the incremental type
encoder is that the absolute position in the rotational angle cannot be
determined, and it is required, in some cases, to provide an additional
separate sensor to detect the absolute position.

[0007]As a solution to this problem, the following system has been
developed. FIG. 16 shows a scale disclosed in Japanese Patent Application
Laid-Open No. H10-318790, in which the transmittance of the pattern on
the scale 1 is varied to enable to detect the absolute position in an
incremental type encoder. In this scale, mark 1a has a transmittance of
1, and transmittances of marks 1b, 1c, 1d . . . gradually decrease.

[0008]FIG. 17 shows changes in signals that occur when a portion of the
scale 1 in which the transmittance varies passes by a sensor in an
encoder having this scale 1. Signals A and B are analogue two phase
signals obtained from the sensor.

[0009]The amplitude of the signals decreases with a gradual decrease in
the transmittance of the mark of the scale 1, and the absolute position
can be detected by detecting this change in the signal amplitude.

[0010]In an absolute position detection unit used in the above described
conventional encoder, in order to detect the signal amplitude, it is
required to sample signals at intervals significantly shorter than a
cycle of the encoder signals obtained.

[0011]Since it is necessary to determine the peak voltage and valley
voltage of the signals based on the result of sampling, a large scale
circuit such as a high speed A/D converter is required to be provided.

[0012]In addition, it is difficult to produce a scale including portions
having different transmittances with reliability, and significant
variations in actual changes in the signal amplitude will be generated.

[0013]An object of the present invention is to provide an origin detection
method for an optical encoder that enables origin detection with a simple
structure by performing computation according to changes in the sensor
signal with movement of a scale.

SUMMARY OF THE INVENTION

[0014]To achieve the above object, according to the present invention,
there is provided an origin detection method for an optical encoder
technically characterized in that the optical encoder has a scale
provided with an optical grating, a plurality of light receiving elements
that is provided in association with the pitch of the optical grating and
movable relative to the scale and a light source that illuminates the
light receiving elements with light through the scale, wherein an
optically discontinuous portion is provided in the optical grating of the
scale, a change of a light beam that occurs over a certain length of
section at the time when a light beam corresponding to the discontinuous
portion passes through the light receiving elements, a change occurring
in that section is detected, calculation is performed, and an origin
position is detected from the result of the calculation.

[0015]According to the origin detection method for an optical encoder
according to the present invention, in detecting the absolute position,
the origin position can be determined by detecting a point of change in
an analogue middle voltage based on a relationship between the number of
units of the light receiving portion and the discontinuous portion of the
scale.

[0016]According to the arrangement of the present invention, since an
encoder signal and an origin signal are synchronous signals obtained from
the same scale, a highly accurate origin signal can be produced.
Furthermore, no additional parts are needed to detect the origin, and
accordingly, encoders having an origin position detection function can be
manufactured at low cost.

[0017]Further features of the present invention will become apparent from
the following description of exemplary embodiments with reference to the
attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 schematically illustrates the structure of an optical encoder
according to a first embodiment.

[0019]FIG. 2 illustrates a pattern of a light receiving portion and a
light-and-shade pattern detected.

[0022]FIG. 5 is a graph showing changes in a signal at a time when a
deficient slit passes by, which is detected by the processing circuit,
and result of calculation.

[0023]FIG. 6 is a graph showing changes in middle voltage.

[0024]FIG. 7 is a graph showing changes in an origin signal which is a
difference that a middle voltage (which is a middle voltage during
changing from Amin to Amax) minus the sixth-previous middle voltage.

[0025]FIG. 8 is a flow chart of an origin detection algorithm.

[0026]FIG. 9 schematically shows the structure of an encoder according to
a second embodiment.

[0027]FIG. 10 is a graph showing changes in a signal at a time when a
deficient slit passes by, which is detected by the processing circuit,
and result of calculation.

[0028]FIG. 11 is a graph showing changes in middle voltage.

[0029]FIG. 12 is a graph showing changes in an origin signal which is a
difference that a middle voltage (which is a middle voltage during
changing from Amin to Amax) minus the sixth-previous middle voltage.

[0030]FIG. 13 is a graph showing changes in middle voltage in a third
embodiment.

[0031]FIG. 14 is a graph showing changes in the middle voltage.

[0032]FIG. 15 is a graph showing changes in an origin signal which is a
difference that a middle voltage (which is a middle voltage during
changing from Amin to Amax) minus the twelfth-previous middle voltage.

[0033]FIG. 16 schematically illustrates the structure of a scale portion
of a conventional optical encoder.

[0034]FIG. 17 shows waveforms of signals output from an encoder when a
portion of a scale in which the transmittance varies is passing by a
sensor.

DESCRIPTION OF THE EMBODIMENTS

[0035]In the following, the present invention will be described based on
embodiments illustrated in FIGS. 1 to 15.

First Embodiment

[0036]FIG. 1 schematically shows the structure of an encoder that uses a
micro roof mirror array as a reflective scale. The encoder has a light
emitting portion 11, a light receiving portion 12 and a scale 13 as a
moving member. Light emitted from the light emitting portion 11 is
reflected by the scale 13 that has a reflexive portion 13a and an
irreflexive portion 13b, so that a light-and-shade distribution pattern
is formed on an array of photodiodes 14 serving as light receiving
elements in the light receiving portion 12 shown in FIG. 2.

[0037]FIG. 1 shows only a part of the scale 13. Actually, the scale 13 is
long in its moving direction and has a region 13a in which reflexive
portions and irreflexive portions are arranged at a certain cycle and a
partial region 13b in which a reflexive portion is absent.

[0038]The scale 13 is not necessarily required to have micro roof mirror
array, but it may be a simple structure having reflexive portions and
irreflexive portions.

[0039]The light receiving portion 12 shown in FIG. 2 has a plurality of
units, each of which includes four photodiodes 14. The length of the unit
including four photodiodes 14 corresponds to the length of one
light-and-shade cycle of the scale 13. The light receiving portion 12
shown in FIG. 2 has six units. By performing computation based on signals
obtained from the four photodiodes 14, two phase sinusoidal signals
having a phase difference of 90 degrees can be obtained.

[0040]High intensity portions La of the light incident on the light
receiving portion 12 from the scale 13 are distributed in a specific
relation with respect to the scale pitch. Thus, there is a high light
intensity portion La in one unit in the light receiving portion 12. In
this embodiment, since the irreflexive portion 13b is provided on the
scale 13, a low light intensity portion Lb is generated among the high
light intensity portions La.

[0041]According to this structure, even when one unit in the light receive
portion does not receive the reflected light due to the presence of the
irreflexive portion 13b on the scale 13, there are several photodiodes 14
that receive the reflected light in the other units, and signals having a
certain decreased amplitude are obtained. In this embodiment, for
example, the light receiving portion 12 is adapted to receive six high
light intensity portions La, and one of the six high light intensity
portions is absent. Therefore, a light quantity equal to five sixths of
the normal light quantity is obtained. Corresponding photodiodes in the
respective units may be interconnected so that an added-up signal is
output, or a circuit that adds up the signals output from those
photodiodes may be provided.

[0042]FIG. 3 is a circuit diagram of a processing circuit in this
embodiment. The processing circuit has current-to-voltage converters
provided downstream of the four photodiodes 14a to 14d in one unit
respectively. The photodiodes 14a to 14d output signals having phase
differences of 90 degrees from one another.

[0043]Signals from photodiodes 14a and 14c and signals from photodiodes
14b and 14d have phase differences of 180 degrees respectively. These
signals are input to analogue amplifiers 21a to 21d. The outputs of the
analogue amplifiers 21a to 21d are input to the plus and minus terminals
of comparators 22a and 22b so as to be binarized. Thus, an A-phase
digital signal DA and a B-phase digital signal DB are output.

[0044]The outputs of the analogue amplifiers 21a to 21d are connected to
differential amplifiers 23a and 23b, and a voltage Vref2 is applied to
the differential amplifiers 23a and 23b. Therefore, an A-phase analogue
signal A and a B-phase analogue signal B in which the voltage Vref2 is
the central voltage in the analogue signals A and B are output. With the
above described circuit configuration, digital signals that change at
central points of the analogue signals are obtained.

[0045]FIG. 4 shows waveforms of signals obtained from this circuit. Since
the digital signals DA, DB are generated at zero crossing points of the
analogue signals A, B, and the phases of the analogue signals A and B are
different from each other by 90 degrees, rising edges and trailing edges
of the B-phase digital signal DB coincide with times at which the A-phase
analogue signal A becomes maximum and minimum.

[0046]Therefore, by sampling the A-phase analogue signal A at timings of
pulse edges of the B-phase digital signal DB, the maximum value and the
minimum value of the A-phase analogue signal A in one cycle thereof can
be obtained. The amplitude of the A-phase analogue signal A and the
middle voltage of the A-phase can be obtained by the following equations
based on the maximum value Amax and the minimum value Amin.

Amplitude=Amax-Amin

Middle Voltage=(Amax+Amin)/2

[0047]FIG. 5 shows a detection signal of the value of the A-phase analogue
signal A obtained by using the light receiving portion 12 in FIG. 2 at
timings of rising and trailing edges of the digital signals DA, DB in a
case where the width of the irreflexive portion 13b on the scale 13 is
equal to the width of one slit.

[0048]Four signals that are sampled at timings of rising and trailing
edges of the digital signals DA, DB are obtained in one cycle.

[0049]For example, in the case of a rotary type scale 13 which generates
one thousand pulses per one rotation, a thousand sets of four signals are
obtained by one unit per one rotation of the scale 13, namely, four
thousand data are detected in total.

[0050]In FIG. 5, the amplitude (Amax-Amin) is relatively small over 6
cycles.

[0051]This occurs when the irreflexive portion 13b of the scale 13 passes
by the light receiving portion 12. Normally, a light-and-shade pattern
corresponding to six pitches is formed on the light receiving portion 12.
In the above case, however, a portion of the scale equal to one pitch (or
the irreflexive portion 13b) does not reflect light, and therefore the
signal amplitude decreases to of that in the normal time.

[0052]In FIG. 5, the middle voltages A or (Amin+Amax)/2 of the A-phase
analogue signal A while the signal value changes from the minimum value
Amin to the maximum value Amax are plotted as black dots.

[0053]FIG. 6 shows the middle voltage in an enlarged manner. The middle
voltage increases or decreases at a moment when the irreflexive portion
13b of the scale 13 passes by the light receiving portion 12. When the
irreflexive portion 13b passes from analogue-signal-A side (passes from
the photodiode 14a toward the photodiode 14d in FIG. 3), a decrease in
the middle voltage occurs. On the other hand, when the irreflexive
portion 13b passes from analogue-signal-A side (passes from the
photodiode 14d toward the photodiode 14a in FIG. 3), an increase in the
middle voltage occurs. However, such signal characteristics can change
depending on the wiring of the photodiodes 14, and the above described
signal characteristics are not always the case.

[0054]In FIG. 6, when the scale 13 moves from the "0" side to the "80"
side of the graph, a decrease in the level of the middle voltage occurs
first, and then an increase in the level of the middle voltage occurs.
Such a decrease and an increase in the middle voltage level occur at
times when the irreflexive portion 13b passes by the edge of the light
receiving portion 12. Therefore, in the case where the light receiving
portion 12 has six units, an increased portion and a decreased portion in
the signal level appear at an interval corresponding to the six pitches.

[0055]FIG. 7 is a graph showing the difference between the middle voltage
data shown in FIG. 6 and the sixth-previous middle voltage data. As will
be seen from FIG. 6, the increased portion and the decreased portion in
the middle voltage appear at an interval of six (that is, the number of
units), and accordingly, the aforementioned difference can show the
position at which the signal changes in an exaggerated manner.

[0056]Thus, the origin position can be determined by specifying a point as
the origin through a signal processing when the difference between the
middle voltage data obtained and the sixth-previous middle voltage data
exceeds a certain threshold level (LV1).

[0057]FIG. 8 is a flow chart of the algorithm according to this detection
method.

[0058]Step S1: Origin detection is started. The scale 13 is moved, and
signals are generated.

[0059]Step S2: The maximum values Amax and the minimum values Amin of the
A-phase analogue signal A are detected by detecting the value of the
A-phase analogue signal A at timings of rising edges and trailing edges
of the B-phase digital signal DB.

[0060]Step S3: The middle voltage of the A-phase analogue signal A is
calculated as (Amax+Amin)/2.

[0062]Step S5: The origin is set at the rising edge of the A-phase digital
signal DA at the time when the pulse value satisfies the condition
"SA> threshold level LV1".

[0063]By setting the origin position at a specific pulse edge of the
digital signal in this way, the origin position can be determined with
high accuracy.

[0064]In conventional methods, the position at which the middle voltage
shows the maximum value Amax or the minimum value Amin is determined,
data are stored over a certain range, and processing such as value
comparison and differentiation of stored data is performed. Therefore,
processing is complex.

[0065]In contrast, in this embodiment, it is sufficient only to simply
check whether or not the difference between the current data and the
sixth-previous data is larger than a certain threshold value. Thus, the
origin position can be determined in a simple manner with reliability.

[0066]Although signal changes in the case where the number of the units of
the photodiodes 14 is six has been described in the foregoing, if, for
example, the number of the units in the light receiving portion 12 is
five, the difference between the current data and the fifth-previous data
should be calculated. In this way, which previous data is to be used in
calculating the difference is determined depending on the number of units
in the light receiving unit 12.

Second Embodiment

[0067]FIG. 9 schematically shows the structure of an encoder according to
a second embodiment of the present invention. The scale 13 in the first
embodiment shown in FIG. 1 is provided with one irreflexive portion or
slit 13b, the scale 13 of the second embodiment is provided with two
irreflexive portions 13b, 13b' arranged continuously.

[0068]FIG. 10 shows a signal waveform of the A-phase analogue signal A
retrieved at pulse edges of the digital signals DA, DB and the middle
voltage between the minimum value Amin and the maximum value Amax when
the analogue signal A changes from the minimum value Amin to the maximum
value Amax in the case where the scale is provided with two irreflexive
portions 13b, 13b'.

[0069]FIG. 11 shows the middle voltage of the A-phase analogue signal in
an enlarged manner. What is different in this graph from the
corresponding graph in the first embodiment is that two successive
decreased portions and two successive increased portions occur in the
middle voltage. As will be seen from FIG. 11, the increased portion and
the decreased portion in the middle voltage level appear at an interval
of six (that is, the number of units), and the portion in which the
reflexive portion is absent includes two sections. Therefore, the
difference between the current data and the sixth-previous data shows a
peak portion that includes two detection points.

[0070]FIG. 12 is a graph showing the difference between the current data
and the fifth-previous data. In the second embodiment, the difference
between the current data and the fifth-previous data is calculated. In
this case, the peak portion of the resultant detection signal includes
one detection point, and the origin position can be determined. Thus, the
origin position can be determined by signal processing as a point at
which the difference between the middle voltage data obtained and the
fifth-previous middle voltage data exceeds a certain threshold level
(LV2).

[0071]In the case where the difference between the current data and the
n-th-previous data is calculated, it is necessary for the calculation
circuit to store n data. Therefore, the larger the number n is, the
larger storage capacity the calculation circuit is required to have. In
the second embodiment, the required storage capacity can be reduced by
providing two irreflexive portions 13b, 13b', and the calculation circuit
can be made simpler.

[0072]In this embodiment, a calculation method for determining the one
origin position per one rotation of the scale 13 has been described.
However, in the case where the difference between the current data and
the sixth-previous data is calculated, an origin signal including two
pulses is obtained per one rotation of the scale 13, and the signal may
be subjected to calculation so that it is used as a zone signal.

[0073]Although in this embodiment, the difference is calculated for data
with an interval equal to "(the number of units)-(the number of
irreflexive portions)+1", the origin position can be determined from the
difference between data with an interval equal to "(the number of
units)+(the number of irreflexive portions)-1".

Third Embodiment

[0074]In the first and second embodiments, the middle voltage is
calculated as (Amin+Amax)/2 based on data in the section in which the
A-phase analogue signal A changes from the minimum value Amin to the
maximum value Amax.

[0075]However, when the moving direction of the scale 13 is reversed,
without paying attention to whether the data are in an interval in which
the signal changes from the minimum value to the maximum value during the
scale rotates in a certain direction when retrieving the data, data may
be sampled during the scale 13 rotates in the reverse direction.

[0076]Therefore, when calculating the value (Amin+Amax)/2 based on data in
FIG. 5, if a minimum value and its immediately subsequent maximum value
in FIG. 5 viewed in the reverse direction of time are adopted (that is, a
minimum value and its immediately previous maximum value are actually
adopted) for the calculation) changes in the middle voltage do not appear
as shown in FIG. 13.

[0077]Therefore, it is necessary to settle the order of retrieving data
to, for example, the order from the minimum value Amin to the maximum
value Amax so that the calculation of the middle voltage (Amin+A max)/2
is performed surely based on data in a section in which the A-phase
analogue signal A changes from the minimum value Amin to maximum value
Amax. Therefore it is necessary to provide means for determining whether
the data that is obtained first is data of the minimum value Amin or data
of the maximum value Amax. This makes the circuit or the calculation unit
complex.

[0078]The third embodiment is intended to eliminate the above descried
problem. The value (Amin+Amax)/2 is calculated from both of data in
sections in which the A-phase analogue signal changes from the minimum
value Amin to the maximum value Amax and data in sections in which the
A-phase analogue signal changes from the maximum value Amax to the
minimum value Amin, and the values (Amin+Amax)/2 calculated from both
types of data are used as input data.

[0079]Furthermore, in the case where the light receiving portion 12
includes six units, the difference between the current data and the
twelfth-previous data is calculated, in contrast to the first embodiment
where the difference between the current data and the sixth-previous data
is calculated.

[0080]FIG. 14 is a graph showing the middle voltage calculated based on
data in both sections in which the A-phase analogue signal A changes from
the minimum value Amin to the maximum value Amax and sections in which
the A-phase analogue signal A changes from the maximum value Amax to the
minimum value Amin. By retrieving the data from both sections, changes in
the middle voltage appear surely.

[0081]FIG. 15 is a graph showing the difference between the current data
and the twelfth-previous data calculated based on the series of data
shown in FIG. 14. In the graph of FIG. 15, the calculated difference
becomes large at the origin. Thus, the origin position is determined by
the processing circuit as a point at which the difference between the
current data and the twelfth-previous data exceeds a certain threshold
level (LV1).

[0082]Therefore, resetting of the counter at the origin position can be
performed, and an origin signal can be output in synchronization with the
A-phase analogue signal A at that time.

[0083]While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is not
limited to the disclosed exemplary embodiments. The scope of the
following claims is to be accorded the broadest interpretation so as to
encompass all such modifications and equivalent structures and functions.

[0084]This application claims the benefit of Japanese Patent Application
No. 2006-257199, filed Sep. 22, 2006, which is hereby incorporated by
reference herein in its entirety.